Light-emitting diodes (LEDs) have been known for use in indicators and selective electronic displays for many years. Many recent advances in the technology of light-emitting diodes (LEDs) has caused increased interest in using LEDs for purposes of illumination and, indeed, made LED arrays the illumination medium of choice for numerous applications such as exterior and interior area illumination and backlighting of display panels due to the efficiency, spectral content, long lifetime, eco-friendliness, mechanical durability, safety and efficiency compared to incandescent, fluorescent, mercury and sodium vapor and arc lighting and the like.
Another important quality of LEDs for many such illumination applications is the capability for full control of light output flux, sometimes referred to as dimming. However, dimming of LEDs presents some problems in the design of power supplies for LED arrays particularly in providing good uniformity of light output of all LEDs in an array and avoiding perceptible flickering consistent with high efficiency of the power supply. For example, driving LEDs individually or in long, series connected strings with individual discrete power supplies is cost prohibitive and generally would require complex cross-regulation to achieve acceptable uniformity of light flux. Also, since power supplies are designed for highest efficiency at a particular voltage and frequency, efficiency is often greatly reduced as voltage is controlled, particularly when that voltage control is achieved by frequency control in resonant power converters. Moreover, Also, since light output flux of LEDs terminates immediately upon interruption of current, duty cycle or pulse width modulation (PWM) must be performed at a switching cycle frequency above about 85 Hz whereas such a problem is not presented by incandescent bulbs which exhibit a decrease in light output flux over the period of filament cooling.
Among known designs of power converters, resonant switching power converters have become popular due to their ability to limit switching losses and electrical stresses during operation as well as providing very high efficiency. Among resonant power converters, so-called LLC resonant converters are becoming increasingly attractive because of their flexibility of application, simplicity, efficiency, the simplicity of their control the ability to deliver a range of voltages and the possibility, although difficult, of providing over-current protection.
Typically, an LLC resonant converter will comprise a pair of switching transistors operated in a complementary fashion and a resonant circuit comprising a capacitor and two inductors. An LLC resonant converter typically operates at a switching frequency near the resonant frequency, f0 of the LLC circuit for highest efficiency. As an electrical load is increased and more power must be delivered, simple sensing and feedback of the output voltage to a voltage controlled oscillator (VCO) can be arranged to reduce the switching frequency and increase the voltage gain to automatically compensate for the increased required power and thus provide good voltage regulation over a wide range of current. By the same token, particular conditions of voltage, current or switching frequency can be sensed and the VCO can be controlled to increase the switching frequency to reduce gain of the power converter and thus provide over-current protection in a very simple and robust manner. However, while steady-state performance of resonant power converters is well-matched to power requirements of LEDs other than loss of efficiency due if switching frequency is used to control voltage, interruption of input or output of power as is necessary for PWM or duty cycle modulation (e.g. for dimming) causes transients in the resonant circuit that may have perceptible adverse effects on light output flux.